A Study to Determine the Abuse Potential of Tozadenant Relative to D-Amphetamine and Placebo When Administered Orally in Healthy, Non-Dependent, Recreational Polydrug Users

The Qualification Phase will be conducted as a single, 4-day visit. Doses will be administered in a randomized, double-blind crossover manner following administration of a standard low-fat meal. Subjects will be dosed with 20 mg of d-amphetamine or matching placebo d-amphetamine on Day 1 and Day 2, approximately 24 hours apart. PD assessments will be conducted before dosing and at time points for up to 8 hours post dosing. Safety assessments will be conducted before dosing and for at least 24 hours following dosing. Data will be reviewed to determine subject eligibility. The last drug administration in the Qualification Phase and the first drug administration in the Treatment Phase will be separated by a washout interval of at least 7 days and not to exceed 28 days. During the Treatment Phase, there will be 6 treatment periods; subjects will receive a single oral dose of each of the following treatments with applicable matching oral placebos in a randomized, double-blind, double-dummy fashion following the administration of a standard, low-fat meal. The following treatments will be administered: * Treatment A: placebo (matched to tozadenant and d-amphetamine) * Treatment B: tozadenant 120 mg * Treatment C: tozadenant 240 mg * Treatment D: tozadenant 480 mg * Treatment E: d-amphetamine 20 mg * Treatment F: d-amphetamine 40 mg Drug administration will occur on Day 1 of each of the 6 treatment periods. PD and PK assessments will be collected during the 24 hours post-dose and safety assessments will be collected during the 36 hours post-dose. Subjects will be discharged on Day 2, after approximately 36 hours post-dose, or remain at the clinical research unit longer (e.g., 48 hours or until the following morning) if there are safety concerns, at the discretion of the investigator or designee. Drug administration in each treatment period will be separated by a washout interval of at least 7 days after the last dose of study drug. Subjects will return for an end-of-study safety Follow-up visit approximately 7 to 14 days after the subject's last study drug dose in the Treatment Phase or following early withdrawal.